The present invention relates to an X-ray CT (Computed Tomography) apparatus, and more specifically to an X-ray CT apparatus which changes an X-ray output of an X-ray tube according to view angles.
There has been known, upon imaging using an X-ray CT apparatus, a method in which only when an X-ray tube is in a prescribed view angle range close to a tissue high in radiation sensitivity, an X-ray output of the X-ray tube is set smaller than a normal level to perform a scan (refer to, for example, Japanese Patent Publication Laid-Open No. 2004-321587 and abstract thereof). According to this method, it is possible to reduce the amount of radiation exposure to the tissue high in radiation sensitivity.
Tissues high in radiation sensitivity include crystalline lenses, thyroid, lacteal or mammary glands, a generative organ, etc. These tissues high in radiation sensitivity and their peripheral tissues are configured approximately in a laterally symmetrical manner as viewed from a subject. For this reason, in general, when the above method is used, a view angle range of an X-ray tube in which the level of an X-ray output is made smaller, is also attempted to be arranged in a laterally symmetrical manner on the basis of a view angle of the X-ray tube corresponding to the direction of the front of a subject, e.g., a view angle at which the X-ray tube is located directly above the subject.
The X-ray output of the X-ray tube has, however, a characteristic in which it is not switched soon even if switch control is started, but changes with a certain amount of short time. A rise characteristic of the X-ray output and a fall characteristic thereof are different from each other. That is, when the X-ray output is raised and lowered, the way of a change in the X-ray output and the time necessary for the X-ray output to change differ. For this reason, even if the view angle range of the X-ray tube in which the X-ray output is set smaller than a normal level, and is arranged in a laterally symmetrical manner, the dose of X rays applied to the subject is not laterally symmetrical with each other. It is thus difficult to reduce the amount of radiation exposure to the tissue high in radiation sensitivity in a laterally well-balanced fashion.
With the foregoing in view, there has been a demand for an X-ray CT apparatus capable of reducing the amount of radiation exposure to a tissue high in radiation sensitivity in a laterally well-balanced fashion.